ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
677 
Table VIII. 
No. of 
Experiment. 
Average effective 
length of 
black in m.m. 
dp 
dp’ 
rZT/T 
(in percentages). 
VIII. 
23-5 
0-98 
-0'41 
IX. 
18-0 
0-90 
-1-18 
X. 
17-0 
0-80 
1-50 
11-0 
0-60 
-0-15 
XI. 
16-5 
0-72 
-0-28 
XII. 
20-0 
0-89 
0-05 
21-0 
0-90 
—0-49 
XIII. 
16-0 
0-77 
-1-59 
16-0 
077 
0-60 
XIV. 
18-0 
o-o 
o-o 
XV. 
24-0 
0-98 
-0-83 
XVI. 
26-0 
1-00 
-0-68 
XVII. 
17-5 
076 
031 
XVIII. 
20-0 
0-87 
115 
XIX. 
24-0 
0-98 
006 
Mean 
—0T3 
The experiments are not all of the same value. Those in which dp/clp' is most 
nearly unity are better than those in which it is small. So many other considerations, 
however, affect the weight of the observation, e.g., the time during which it lasts, 
&c., &c., that we have made no attempt to discriminate between them. 
The range of the above observations is from the green of the fifth order to the black 
of the first. In Experiment XIX. a film almost entirely black was balanced against 
another, the colour of which was G(5, 6). This tint corresponds to a thickness of 
about 1350 X 10~ G mm. The value of c/T/T was in this case unusually small. 
In all these experiments the black extended over the film quietly and regularly. At 
times the black is formed with something like a convulsion. Not only does it spread 
with extraordinary rapidity, but the edge is violently disturbed and large patches rise 
through the coloured part of the film. Whenever this occurs the film breaks before 
long, but in four cases we were able to obtain measurements before rupture. We are 
not able to produce this phenomenon at will, but the few observations we have been 
able to make on it are in agreement among themselves, and we therefore describe them, 
as we think they make it necessary to generalise our results with very great caution. 
In each case we give the colours and value of 8 from observations taken immediately 
before and after the sudden increase of the black surface. A rapid increase was 
observed in Experiment XIY., but the general character of the mode of formation was 
normal. The increase tabulated in the last column is the change in 8 taken positive 
when the thinner film bulged. 
